ABSTRACT/PROJECT SUMMARY In this proposal we explore the transition from preclinical to symptomatic Alzheimer disease (AD) by incorporating the novel tau imaging tracer ([18F]-AV-1451) within the biomarker-rich protocol of the Dominantly Inherited Alzheimer Network (DIAN). We leverage the existing infrastructure of DIAN and a collaboration with Avid Radiopharmaceuticals to initiate tau positron emission tomography (PET) imaging in this unique cohort. DIAN provides fundamental support to the hypothesis that AD consists of a preclinical stage in which accumulation of beta-amyloid (A?) plaques and tau neurofibrillary tangle (NFT) gradually lead to neuronal dysfunction and cognitive impairment. However, key gaps remain in our understanding of the temporal and spatial interactions that occur between A? and tau during the transition from preclinical to clinical symptoms. This proposal answers these fundamental questions through three aims. Aim 1 studies the temporal dynamics of tau deposition (using AV-1451) in relation to estimated years to symptom onset (EYO) and existing biomarkers in DIAN (including cerebrospinal fluid, neuroimaging, and cognitive performance). Aim 2 studies the spatial (both local and distributed) changes of tau deposition (using AV-1451). Aim 3 studies the relationship between in vivo tau deposition (using AV-1451) and neuropathology. Unique to autosomal dominant AD (ADAD), the young age of the DIAN participants eliminates overlap with potential age-related neuropathology or tauopathy (PART). Our overall hypothesis is that conversion from cognitively normal to symptomatic AD can be accurately predicted by neuroimaging biomarkers, in particular by AV-1451 PET. Results from this proposal are fundamental for our understanding of PET tau as not only a diagnostic indicator of disease but also a therapeutic marker to evaluate the clinical efficacy of future interventions in ADAD.